4 APPLICATION OF COLD FORMED SECTIONS IN BUILDING
4.9 Frameless structures
Over the last 20 years, there have been important developments in “frameless”
construction, where the members and the cladding interact by “stressed skin”
action. The main structural configurations that have been used are the folded plate roof[16], the truncated pyramid and the barrel vault.
The folded plate roof behaves as a series of inclined beams spanning between end frames. The fold-line members (cold formed angles) absorb the axial thrusts and tensions, and the sheeted web is in pure shear. From the point of view of aesthetics and structural performance the slope of the roof would normally be between 30 and 45E (to the horizontal) and most efficient spans are between 15 m and 25 m.
The truncated pyramid roof, as shown in Figure 4.14, comprises a compression ring and a tension ring with hip members transferring the loads between. The individual sheeted panels are prefabricated and they can be bolted together and lifted into place in a few hours. Gutter outlets are placed along the valleys and down the hollow section columns.
Figure 4.14 Example of truncated pyramid roof using cold formed steel sections
4.10 Connections
The common types of fixing between cold formed sections, and between sections and sheeting, are:
Type Usual Application
(a) Bolts Connecting cold formed sections.
(b) Self-tapping screws Fastening sheeting to sections (< 6 mm thick) or sheeting to sheeting at sidelaps.
(c) Blind rivets Fastening sheeting to sheeting at sidelaps.
(d) Powder actuated pins Fastening sheeting to members (>6 mm thick).
(e) Spot welding Factory joining of thin steel.
(f) Puddle welding Site welding of sheeting to sections.
(g) Clinching Usually factory installed by press-joining.
(h) Self-piercing rivets Usually factory installed.
(i) Nailing Site installed using special nails.
These different types of fixing are reviewed as follows:
(a) Bolts: Bolt holes can be punched in cold formed sections; the connections between members are usually arranged so that the bolts are loaded in shear.
In almost all cases, the resistance of the connection is determined by the bearing resistance of the thinner steel section, rather than by shear of the bolt.
Countersunk bolts can be located in recessed holes punched into the sections. In this way, the bolt head does not protrude and does not affect the fixing of the plasterboard or other lining (see Figure 4.15).
Figure 4.15 Countersunk bolts between wall elements
Autoform ends may also be formed during the cutting and punching process;
these facilitate bolting (see Figure 4.16).
Figure 4.16 Creation of autoform ends to C sections
In BS 5950-5, Clause 8.2.5.2 the shear resistance of bolted connections is given as:
Pu = 2.1 d t Ys for t # 1 mm (14)
or Pu = (1.65 + 0.45 t) d t Ys for 1 mm < t # 3 mm (15) where:
d is the diameter of bolt (mm)
Ys is the design strength of steel in thinner plate (N/mm2) t is the thickness of steel in thinner plate (mm)
These shear resistances assume that the bolt end distance is at least 3d and that washers are used under both the head and the nut. The resistances are greater than the equivalent values in BS 5950-1, because of build up of deformed steel in front of the bolt as the thinner section fails in bearing. They also include a partial safety factor, given by the ratio of the ultimate to the yield strength of the steel (approximately 1.4).
(b) Self-tapping screws: Self-drilling self-tapping screws are commonly used for connecting thin steel components. A selection of the screws and the drill that may be used is shown in Figure 4.17. The “drill” part of the screw forms a hole in the steel plate and the “tapping” part forms the thread. This is a single operation and gives a relatively strong and stiff form of attachment.
“Thin-thick” and “thin-thin” attachments may be made depending on the length of the screw. The diameter of the screws is in the range of 4.2 to 8.0 mm, the most common size being about 6 mm for thin-thick connections.
The shear resistance (including partial safety factors) may be obtained from Appendix A of BS 5950-5.
Figure 4.17 Different forms of self-drilling self-tapping screws and the standard drill
Typically, for a 6 mm diameter screw through 1 mm thick steel, the shear resistance (including partial safety factors is 3.5 kN (thin-thick fixings) and 2.2 kN (thin-thin fixings). Premature failure of a fixing by pull-out should be avoided by careful detailing.
Fixings have also been developed for stand-off applications where insulation materials or timber are to be attached (Figure 4.18).
Figure 4.18 “Stand-off” type fixing for soft insulation materials
(c) Blind rivets can be in aluminium or alloyed metal (often termed “monel”).
They are fitted from one side into predrilled holes and a mandrel is pulled by a special tool so that the rivet expands into and around the hole. These rivets are commonly of 2.4 to 6.3 mm diameter (dependant on the hole diameter).
It is a relatively firm form of attachment with good pull out resistance and is useful for thin-thin attachments, e.g. seams in profiled decking. Again, Appendix A of BS 5950-5 can be used to obtain the shear resistance.
The “huck” bolt is a similar system, but is used for thicker materials. As for the blind rivet it is fitted from one side in a pre-drilled hole. Tension is applied to the stem of the bolt by a special tool and a malleable ring is pushed to precompress the plates to be attached. When the correct tension has been applied the outer part of the stem breaks off.
(d) ‘Shot-fired’ pins (more correctly powder actuated fasteners) are often used when fastening thin to thick steel. They are usually of 4.2 mm diameter and the powder of cartridge is selected for the thickness of the steel. The shear resistance of this type of fixing is given in Appendix A of BS 5950-5 as:
Pu = 3.2 t d Ys (16)
The enhanced resistance, relative to a bolt of the same diameter, is due to the tight fit and the precompression of the steel around the fixing.
(e) Spot welding is a technique used mainly in the factory. An arc is created between the tips of the welding tool on either side of the steel elements to be attached. It is therefore most appropriate where the welding tool can be supported and can be moved easily into place to form the weld.
The shear resistance of a spot weld (in N) is given in BS 5950-5, Clause 8.5.3 as:
Pu = 2.7 t d Ys (17)
(f) Puddle welding is a common site technique in North America. An arc is created between the manually held welding rod and the bare metal. In order to weld through to the lower sheet or base steel, a hole is often cut in the upper sheet and a weld formed around the cut edges.
(g) Clinching is a mechanical process in which two pieces of thin steel are pressed together to form a local embossment in the surface of the steel.
Clinched connections are effective in shear but are ineffective in tension.
(h) Self piercing rivets have been introduced into the Surebuild system. The machinery requires sufficient access to make the connection, and therefore short ‘gusset’ pieces are often introduced to facilitate these connections between wall studs.
(i) Nailing of secondary timber elements to thin steel sections or sheeting is feasible by using a “twist-grip” nail. This is practicable where the steel is 0.7 mm to 1.0 mm thick; thicker materials require too much effort to pierce;
thinner materials offer too little pull-out resistance.
4.10.1 Simple connections
Conventionally, connections between cold-formed steel sections comprising two bolts per member are considered as simple (shear resisting) connections. Hot rolled angles are often used, but cleats of this size are generally over-sized for this application.
In a recent experimental investigation[17], it was demonstrated that cold-formed steel web-cleats may be used for improved buildability. Both bolts and self-drilling self-tapping screws may be used as fasterners for cold-formed steel web cleats, and they are simple and easy for site installation. Typical shear resistances of simple connections with cold-formed steel web cleats range from 9 kN to 20 kN, depending on the material and geometry of the cold-formed steel web-cleats.
A set of design rules is available[17] to assess the shear resistance of simple connections with cold-formed steel web cleats for different connection configurations, depending on the practical spatial orientation of members.
Detailing rules on the minimum and the maximum dimensions for the position of fasteners should be observed.
4.10.2 Moment connections
In modern roof systems, the purlin-rafter connections between cold-formed steel sections are highly engineered for the good structural performance of the continuous purlin members. A number of different connection configurations with sleeves or overlaps have been developed in various proprietary systems, which offer partial continuity along the purlins, thereby achieving considerable economy. Purlin-rafter connections are usually cleated connections in which the bolts are designed to resist shear.
For beam-column connections, only a limited range of proprietary systems is available. Mechanical enhancement may be used to increase the moment resistance of the connections by mobilising bearing actions between the profiled webs and profiled gusset plates. (A good example of this is the Swagebeam system.)
An experimental investigation on lipped C sections with hot rolled steel gusset plates[17], beam-column connections and also portal frames was carried out.
The webs of the back-to-back C sections were connected by bolts to the gusset plate. The maximum moment that was resisted by these nominally simple connections was up to 85% of the bending resistance of the connected members.